This invention relates to x-ray detectors comprising electrically conductive plates disposed in an ionizable medium such as xenon gas, and more particularly, it relates to plate shapes which increase the quantum detection efficiency.
Ionization chambers are commonly used for detecting x-ray photons and other ionizing radiation. X-ray photons interact with atoms of a heavy detector gas to produce electron-ion pairs. X-ray photons are, generally, absorbed by a gas atom which emits a photoelectron from one of its electronic levels. The photoelectrons move through the gas, interacting with other ionizing gas atoms, to produce a shower of electrons and positive ions which are collected on suitable electrodes to produce an electric current flow. If such electron ion pairs are produced in a region between two electrodes of opposite polarity, they drift along electric field lines to the electrodes and yield an electric current. The electric current that flows between the electrodes is a function of the total number of x-ray photons interacting in the vicinity of the electrodes. In this way, the magnitude of the resulting electric current is a direct measure of the intensity of an x-ray beam impingent between the electrodes.
X-ray detectors of the present kind typically comprise spaced apart electrically conductive plates typically comprising a high Z (i.e., high atomic number), electrically conductive material such as tungsten. These plates are typically disposed in a pressurizable housing having means therein for the application of electrical potentials to the plates. The housing typically comprises, at least in part, a low Z material, such as aluminum, so that it is relatively transmissive to the impinging x-ray beam. Within the housing the parallel conductive plates are immersed in a high pressure gaseous ionizable medium such as xenon. While other gases may be employed as the ionizable medium, xenon is preferred because of its relatively high atomic number, its ionizability in the presence of x-rays, its inertness, and its fluid nature even at relatively high pressures such as 25 atmospheres, which pressure is typical of those employed to increase the absorption of x-rays. Such x-ray detectors are described, for example in U.S. Pat. No. 4,047,040 and in U.S. Pat. No. 4,047,041, both issued Sept. 6, 1977 to the inventor herein and assigned to the same assignee as the present invention. Both of these patents are incorporated herein by reference. These x-ray detectors are particularly useful in computerized tomographic applications involving the imaging of internal body organs of patients exhibiting a variety of symptoms and conditions. Accordingly, to reduce the x-ray level to which patients are exposed, it is desirable that the x-ray detector operate in an efficient manner. Additionally, in order for the radiologist and/or physician to render an appropriately accurate diagnosis, it is highly desirable that the resolution of the resulting tomographic image be as high as possible and in the case of the ionization detectors described herein, the increased resolution is obtained through a closer spacing of the detector plates.
However, a simple shrinking of the distance between the detectors plates produces certain undesirable results. In particular, the plates have a finite thickness which may be as small as approximately 6 mils. As the distance between detector plates is decreased, a larger and larger fraction of the volume into which the x-ray beam is directed is taken up by the detector plates which absorb the x-ray beam energy without producing any detectible current. Thus, as the detector resolution is increased, the thickness of the plates must also be decreased so as to provide a maximum volume of xenon gas for the production of photoelectrons. However, as the thickness of the plates is decreased, there is an increased tendency for the production of microphonic noise which can degrade the resultant image quality. This microphonic noise arises as the result of the various room and machine vibrations being transmitted to the detector plates which, if they are decreased in thickness, have a lower stiffness and mass and correspondingly increased vibration amplitudes.
In spite of these potential problems with microphonic noise, there are several significant advantages arising from x-ray detectors of the present kind, not the least of which is their high degree of efficiency requiring low patient x-ray exposures. Additionally, the gaseous ionization medium fills the geometry of the parallel plate configuration. This is to be contrasted with the use of solid crystalline bodies as x-ray detectors since such crystals must be precisely machined to fill the space between tungsten plates which are still necessary in such detectors to prevent cross talk between adjacent detector cells. Additionally, the xenon as employed in a housing as described above completely fills the spaces between the plates in a uniform manner. This uniformity insures a minimal tracking error, that is, the error that results when the responses of adjacent cells are not consistent in the level of current produced for a given x-ray intensity level. Additionally, ionization detectors of the kind described herein are significantly less expensive to manufacture than those detectors which employ relatively large bodies of scintillation crystal material.